Reference is now made to FIG. 1 showing the configuration of a conventional tunneling field effect transistor (TFET) 10 device. A semiconductor substrate 12 is lightly doped with a first conductivity type (in this example, n-type) dopant. The semiconductor substrate may, for example, be made of a silicon material, and can be either a bulk substrate or a silicon-on-insulator (SOI) substrate. A source region 14 and a drain region 16 are provided in the substrate 12 on each side of a channel region 18, wherein the channel region is made of the lightly doped first conductivity type semiconductor material. Unlike a convention MOSFET device, where both the source and drain are doped with the same conductivity type dopant opposite from that used for the channel (for example, p-type), the TFET is constructed such that the source region 14 is heavily doped with the second conductivity type (in this example, p-type) dopant and the drain region 16 is heavily doped with the first conductivity type dopant. A gate oxide layer 20 is provided over the channel region 18, and a gate electrode 22 is provided over the gate oxide layer 20.
FIG. 2A shows a graph illustrating operation of the TFET device in the OFF state where the applied gate voltage produces band bending that is insufficient to allow tunneling. In this state, the leakage current of the transistor is very low because any such leakage is due only to the drift of minority carriers. FIG. 2B shows a graph illustrating operation of the TFET device in the ON state where the applied gate voltage induces sufficient band bending to produce a reduction of the tunneling barrier width. This leads to band-to-band tunneling (BTBT) of electrons (reference 24) from the valence band of the source region 14 to the conduction band of the drain region 16.
Those skilled in the art further recognize that the provision of germanium (Ge), or to a lesser degree silicon-germanium (SiGe), as the semiconductor material used in the p++ doped source region 14 can improve the tunneling current and the ratio of on-current (Ion) to off-current (Ioff). This is shown in FIGS. 2A-2B by the solid lines for the conduction and valence bands for germanium as compared to the dash-dot lines which show operation using a p++ doped silicon (Si) source region. The increase in the valence band gap (Ev) resulting from the use of a germanium source region 14 further decreases/narrows the tunneling barrier width.
Notwithstanding the foregoing improvement in performance, there continues to be a need in the art to provide a TFET device exhibiting improved drive current capabilities.